Plural compartment mixing vials like those described in Lockhart U.S. Pat. Nos. 2,695,614, Bujan 2,908,274 and Sponnoble 3,464,414 have found widespread application in packaging of medicaments and other substances which, by reason of their instability, must be mixed shortly before use. In such devices, a lower compartment may contain a pulverulent material while a liquid is contained in an upper compartment divided therefrom by a divider plug seated in a constriction in the vial. When a stopper or "piston plug" in the neck of the vial is despressed, hydraulic pressure is transmitted through the liquid to unseat the divider plug, whereupon mixing occurs. Heretofore, the piston plug has been retained in the neck of the bottle by a metal ferrule which is rolled or crimped about a bead formed on the neck after the contents of the vial have been added and the stopper inserted. While that means of plug retention has proved satisfactory in some respects, it suffers the disadvantage of permitting inadvertent depression of the piston plug with consequent premature mixing of the vial contents--i.e., it is not "tamper proof". Again, the rolling process by which the ferrule is crimped about the neck of the vial requires that the latter be formed to close tolerances to minimize breakage and consequent contamination of the vial contents during that operation. In spite of the attention heretofore paid to dimensioning the vial for satisfactory ferrule application, scrap arising from breakage during that operation has remained a problem.